1. Field of the Invention
The present invention relates to an optical head for recording and reproducing data to and from an optical recording medium.
2. Description of Prior Art
An optical disk has attracted attention as a data storage device of a large capacity. Devices employing optical disks as recording media as well as software environments thereof, have been proposed, developed and supplied commercially for applications to a large variety of technical fields. On the other hand, so-called multi-media data processing devices have also been developed in order to deal applications developed in a wide range as an integrated system. Then, it is important to develop an optical head which can access optical disks for various optical characteristics and for various purposes.
Various types of such optical heads for driving an object lens have been proposed. For instance, Japanese Patent laid open Publication 6-333255/1994, and especially FIG. 16 therein, discloses an optical head having two object lenses mounted to a single lens holder. The coma aberration of the object lens along an axis thereof is changed by a moving a magnetic yoke, and the coma aberration of the whole optical system in connection with one of the object lenses for an optical recording medium of a specified thickness is adjusted to become small. Thus, because the coma aberration of the whole optical system becomes small, reading and writing are performed without degrading signals for the optical recording medium.
It is required to use another object lens when data is recorded to and reproduced from an optical recording medium of a different thickness. However, coma aberration of the object lens is different depending on manufacturing conditions thereof, and the magnitude or direction of the coma aberration is also different depending on setting angles of the object lenses relative to a reference surface of the lens holder. Therefore, it is a disadvantage that recording and reproduction are not always good due to degradations of signals for an optical recording medium of the different thickness. Thus, in order to realize good recording and reproduction for optical recording media of various thickness with this arrangement, it is required to decrease manufacturing errors of various lenses or optical parts very much and to improve the setting accuracy of each of the optical parts. Thus, the whole system becomes quite expensive.
In an optical head shown in Japanese Patent laid open Publication 9-120027/1997, a grating is formed on a first surface of an aspherical object lens, in which aberration is corrected such that light of zeroth-order diffraction and of first-order diffractions are respectively condensed on two types of substrates of different thickness of optical disks. The whole object lens is tilted in order to correct coma aberration generated in the optical system in a manner that the tilt angle thereof with respect to the substrates of different thickness become identical.
However, light incident on the object lens is split through diffraction, and the amount of light contributing to a spot on the optical recording medium becomes smaller than that for an ordinary object lens without a grating. Thus, the intensity of the light beam of a laser diode used as a light source becomes large on recording, and the performance is degraded due to increase in heat of the laser diode or a cost thereof becomes high.
For an optical head for an optical disk, it is also required to meet a demand for high densification of optical disks. To achieve this task, it is important to decrease the diameter of a light spot used for recording and reproducing in order to decrease aberration. Therefore, it is required to reduce coma aberration as small as possible by setting the optical axis of the object lens in the optical head vertically with respect to the optical disk.
However, when tilt of the disk surface, that is, so-called disk tilt, happens due to reasons such as deformation of the optical disk, the degraded vertical relationship between the optical head and the recording surface of the optical disk, so that coma aberration is generated in the light spot and the object lens does not have its original performance. Thus, a control means will be required for decreasing coma aberration besides conventional focusing and tracking control. There have already been suggested several types of driving devices for object lens provided with such a control means.
In a conventional optical disk device for tilt control, an optical recording medium is set on a disk driving means, and an optical head is movable freely in radial directions of the optical recording medium. A tilt control motor moves the optical disk device 24 to change its tilt in radial directions of the optical recording medium. The tilt is detected with a sensor.
However, the radial tilt servo explained above is not suitable for rapid response because the whole pick-up optical system is tilted. However, since the tilt of the disk in radial directions is mostly a gentle, simple warp in the surface extending from the internal circumference to the external circumference, no rapid response is required. Then, this is not a problem previously.
However, in a system dealing with signals from optical disks of high density, tilt in tangential directions may immediately lead to degradations in signal qualities. Then, it is also required to correct tangential tilt in addition to the above-mentioned radial tilt. Since frequency components of tilt in tangential directions are higher than the revolution cycle of the disk, the servo is required to have response performance much higher frequencies than compared to cases of tilt in radial directions.